class Solution {
    int[] tmp;
    public int reversePairs(int[] nums) {
        int n = nums.length;
        tmp=new int[n];
        return mergeSort(nums,0,n-1);
    }
    public int mergeSort(int[] nums,int left,int right){
        if(left>=right) return 0;
        int ret=0;
        int mid=(left+right)/2;
        ret+=mergeSort(nums,left,mid);
        ret+=mergeSort(nums,mid+1,right);
        int cur1=left,cur2=mid+1,i=0;
        while(cur1<=mid){
            while(cur2<=right&&nums[cur1]/2.0<=nums[cur2]) cur2++;
            if(cur2<=right){
                ret+=right-cur2+1;
                cur1++;
            }else{
                break;
            }
        }
        cur1=left;cur2=mid+1;
        while(cur1<=mid&&cur2<=right){
            tmp[i++]=nums[cur1]<=nums[cur2]?nums[cur2++]:nums[cur1++];
        }
        while(cur1<=mid) tmp[i++]=nums[cur1++];
        while(cur2<=right) tmp[i++]=nums[cur2++];
        for(int j=left;j<=right;j++){
            nums[j]=tmp[j-left];
        }
        return ret;
    }
}